1. Field of the Invention
This invention relates to an electrophotographic photosensitive medium having a clearcoling layer on a photoconductive insulating binder layer.
2. Description of the Prior Art
Heretofore, there have been known photosensitive members having a photoconductive insulating binder layer containing photoconductive materials dispersed in a binder. In such photosensitive members, the ratio of the photoconductive materials to the binder directly determines the characteristics of the photosensitive member. When a large proportion of binder is used, the electric resistance of the photoconductive insulating binder layer becomes so high that the photoconductive characteristics are lowered and thereby the photosensitive member can not be practically used for electrophotography. Therefore, the binder is usually used at a proportion less than 50% by weight based on the photoconductive material. However, in such case the resulting photoconductive insulating binder layer contains a lot of interstices and is porous, and the surface is uneven. Such state of a photoconductive insulating binder layer adversely affects the characteristics of the layer. For example, the photoconductive material directly appears at the surface of the photoconductive insulating binder layer and is subjected to various types of mechanical, physical and chemical damage.
For example, since transferring paper, cleaning blades and rollers always contact the surface of the photoconductive layer in the transferring step, cleaning step etc., the surface is scratched or abraded. Further, the photoconductive layer is adversely subjected to temperature, humidity, light, dust etc., and when dry developers are used, the developers fill the dent portions on the uneven surface and can not be easily cleaned. In addition, when a liquid developer is used, the photoconductive insulating binder layer is deteriorated by an organic solvent used as a dispersion medium and the deterioration is larger as the proportion of the binder is less.
Electrophotographic photosensitive members having an electrically-insulating layer on a photoconductive insulating binder layer have been widely known which are very useful for electrophotographic processes comprising charging an electrically-insulating layer surface to impart electric charge and forming electrostatic latent images due to the electric charge, for example, the processes disclosed in U.S. Pat. No. 3,666,363 and U.S. Pat. No. 7,734,609.
In the above mentioned electrophotographic photosensitive members, the electrically-insulating layer as a surface layer serves as a protective layer so that many of the above mentioned troubles can be eliminated.
However, the adhesion between the electrically-insulating layer and the photoconductive insulating binder layer is not always good, and in general, the less the proportion of the binder, the weaker the adhesion. Thus, when the adhesion is not strong, the previously mentioned troubles can occur again.
For the purpose of improving this adhesion, an electrically-insulating material is dissolved in an appropriate solvent to produce a liquid coating material for forming an electrically-insulating layer and coated on the surface of a photoconductive insulating binder layer, or an electrically-insulating sheet is adhered to the surface of a photoconductive insulating binder layer with an appropriate adhesive. However, such methods have the following disadvantages. That is, for the purpose of not deteriorating characteristics of the electrophotographic photosensitive member, a binder of less than 50% by weight is used to form a photoconductive insulating binder layer, and an electrically-insulating layer material is applied to the surface of the resulting photoconductive insulating binder layer. In this case, the electrically-insulating layer material penetrates into the interstices in the photoconductive insulating binder layer and fills the interstices so that photoconductive characteristics of the photoconductive insulating binder layer are lowered to a great extent, or a solvent for the electrically-insulating layer material penetrates into the interstices of the photoconductive insulating binder layer and thereby loosens the binding by the binder and as the result, characteristics of the electrophotographic photosensitive member are lowered and in addition, air bubbles from the photoconductive insulating binder layer enter the electrically-insulating layer upon forming said electrically-insulating layer resulting in deterioration of dielectric strength at a portion where the air bubble is present and formation of pin holes.
Further, electrostatic latent images forming characteristics upon charging a surface of an electrically-insulating layer and forming electrostatic latent images based on the electric charge largely depend on an interfacial state between the electrically-insulating layer and the photoconductive insulating binder layer as well as characteristics of the electrically-insulating layer itself. For example, when a photoconductive insulating binder layer possesses characteristics of n-type semiconductor and positive electric charge is given, the positive electric charge is charged on the electrically-insulating layer simultaneously with injection of negative charge from the base plate side and the negative charge is trapped at a portion near the interface between the electrically-insulating layer and the photoconductive insulating binder layer. The negative charge thus trapped attracts the positive charge on the electrically-insulating layer through the electrically-insulating layer resulting in substantially charging positive charge on the electrically-insulating layer. In this case, if the surface of the photoconductive insulating binder layer is uneven, or said layer itself is porous, the electric charge thus bound becomes non-uniform, and electric charge on the corresponding electrically-insulating layer surface also becomes non-uniform and as the result, the electrostatic latent images formed based on such state of electric charge can not be good and if the electric charge is so non-uniform, the resulting electrostatic latent images can not be practically used.
When the surface of the photoconductive insulating binder layer is uneven and the binder layer is porous, the electrically-insulating layer formed on the surface of the binder layer can be neither of uniform thickness nor of smooth surface. This deteriorates electric charging state to a great extent as mentioned above.
When an adhesive is employed, the results are almost similar to above. That is, when an electrically-insulating layer sheet is adhered to the surface of the photoconductive insulating binder layer with an adhesive regardless of a solvent type or non-solvent type, the adhesive component itself or the solvent penetrates into the interstices of the photoconductive insulating binder layer so that undesirable results similar to the case of coating an electrically-insulating layer material directly to the photoconductive insulating binder layer. Further, upon adhering the electrically-insulating layer with an adhesive, a pressure is applied from the electrically-insulating layer sheet side or the photoconductive insulating binder layer side so as to avoid wrinkles and keep the thickness of adhesive between the two layers uniform, and therefore the penetration is more accelerated.
For the purpose of solving those problems in the prior art electrophotographic photosensitive member, there has been proposed to coat the surface of a photoconductive insulating binder layer surface with a clearcoling layer by U.S. Ser. No. 391,761 filed Aug. 27, 1973 abandoned and German Patent Publication (DAS) No. 2344777. These inventions relate to an electrophotographic photosensitive medium comprising a photoconductive layer composed of inorganic photoconductive particles dispersed in a water-insoluble binder, a clearcoling layer on the photoconductive layer and composed of organic solvent-resistant, water-soluble, cohesive materials and an electrically-insulating layer on the clearcoling layer. In the inventions, it is disadvantageous that a material of particularly limited conditions such as organic solvent-resistant, water-soluble, and cohesive material should be used as the clearcoling layer. In addition, since the clearcoling layer material is water-soluble, apts to absorb moisture. Such hygroscopic property is not so serious a problem when the member has an electrically-insulating layer, unless the member is used under particularly severe conditions. However, in case of an electrophotographic member having no electrically-insulating layer, the clearcoling layer is directly exposed to outside atmosphere so that if the member is used in a highly humid atmosphere for a relatively long time, the hygroscopic clearcoling layer absorbs moisture and the electrostatic latent images formed thereon are disturbed (lowering of electrostatic contrast) resulting in obtaining poor transferred images.